1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer, flexible substrate, or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
It will be appreciated that, whether or not a lithographic apparatus operates in stepping or scanning mode, it is vital that the patterned beam is directed onto the appropriate target portion of the substrate surface. In many circumstances multi-layer structures are built up on the surface of the substrate as a result of a sequence of lithographic processing steps. It is of course vital that the successive layers formed in the substrate are correctly in register with each other. Thus, great care is taken to ensure that the position of the substrate relative to the beam projection system is accurately known.
Diffraction effects can cause problems in lithographic apparatus as the patterned beam is not a simple reproduction of a pattern-imparting device, such as a mask or array of controllable elements, but rather is the resultant of a series of diffraction components propagating from the pattern-imparting device. For example, a projection beam propagating from a mask made up of a series of slits of equal width separated by opaque strips of equal width to the slits will be the sum of a series of odd-number diffraction components. The first order components alone will produce a sine wave intensity pattern rather than the square wave, which would result in the absence of any diffraction effects. The greater the number of components collected by the optical system, the closer the resultant intensity pattern will be to the square wave. In practice, however, in many lithographic systems adequate results can be achieved relying only on the first order components because of a “threshold” effect.
The threshold effect arises in lithographic apparatus that is used to expose resists that are fully converted chemically if exposed to light above a predetermined threshold intensity, but are not chemically converted to a significant extent if exposed below that threshold. In the case of the mask discussed above, if only first order components are collected all that is required is that the “sine wave” equivalent of the “square wave” pattern delivers the appropriate threshold exposure across the target region of the substrate. Some overlap of low intensity components of the projection beam outside the target region will not cause the resist outside the target region to be developed. It is therefore conventional practice to provide illumination systems in lithographic apparatus that collect only first order diffraction components from the patterning system. This has the considerable advantage of reducing the required size of lens components in the illumination system.
In microlens array (MLA) imaging systems, a projection system is provided which includes an array of lenses arranged such that each lens in the array projects a respective beam of light towards a substrate to be exposed. Individual beams delivered to respective lenses of the microlens array by a beam expander are directed towards respective parts of the substrate. The lenses of the array may focus the individual beams directly on the substrate, or may direct the beams towards the substrate through a further projection system located between the array and the substrate. The individual beams may be generated by a patterning device made up of, for example, an array of mirrors with one or more of the mirrors being positioned so as to reflect radiation to a respective lens of the array. The patterning device inevitably has a periodic physical structure, and this periodic structure will cause the generation of diffraction components. Conventionally, however, higher order diffraction components had not been thought significant in MLA imaging systems, and it has been thought sufficient to collect at most only first order diffraction components from the patterning device.
Problems have been encountered with such MLA systems with the appearance of “ghost” patterns of exposure, indicating the delivery to one lens of the array of a part of the projection beam intended for another lens, but these problems have been attributed to either misalignment between the projection beam and the lens array or optical aberrations in the illumination system.
Therefore, what is need is a system and method that reduce ghosting effects when a microlens array is utilized in a lithography tool.